1. Field of the Invention
The present invention relates to a rotary anode type X-ray tube, particularly, to a rotary anode type X-ray tube in which the rotary shaft is supported by a dynamic slide bearing.
2. Description of the Related Art
The conventional rotary anode type X-ray tube is disclosed in Japanese Patent No. 3,139,873 and U.S. Pat. No. 5,838,763 and, thus, is already known to the public. In the rotary anode type X-ray tube disclosed in Japanese Patent No. 3,139,873, an electron beam generated from the cathode is impinged on a rotary anode that is rotated as a target so as to cause X-rays to be emitted from the rotary anode. The rotary anode is fixed to a cylindrical rotary structure, and the rotary shaft of the rotary structure is supported in a rotatable condition, by a dynamic slide bearings arranged between the rotary shaft and a stationary shaft. The stationary shaft is fixed to and supported by a supporting-fixing section arranged within a vacuum envelope so as to extend within the vacuum envelope. A cylindrical rotary structure having a heavy rotary anode mounted thereto is fitted to the tip of the stationary shaft with the dynamic slide bearings interposed therebetween.
The rotary anode type X-ray tube having a cantilever beam structure described above is fixed to a gantry of a CT apparatus. The gantry is rotated around a subject to be diagnosed so that the X-ray tube is moved around the subject. A centrifugal force is imparted to the rotary anode type X-ray tube in accordance with rotating movement of the rotary anode type X-ray tube. Thus, a particularly large centrifugal force is imparted to a heavy rotary anode type X-ray tube containing an alloy of a heavy metal as a main component. The centrifugal force applied to the rotary anode is imparted to a rotary structure, and the rotary structure imparts a large bending moment to the supporting-fixing section. As a result, supporting-fixing section and the stationary shaft are bent about the supporting-fixing section so as to bring about displacement of the rotary anode. Such being the situation, a relative slight movement is generated between the rotary anode and the cathode so as to cause the electron beam to be defocused and to be incident on the rotary anode. Alternatively, the focal point of the electron beam is shifted. As a result, it is possible for the rotary anode type X-ray tube to fail to emit an X-ray with a high accuracy. It should also be noted that the rotation of the rotary structure is rendered unstable so as to markedly shorten the life of the rotary anode type X-ray tube.
In the conventional rotary anode type X-ray tube having a cantilever beam structure, the rigidity of each of the stationary shaft, the supporting-fixing section, and the vacuum envelope is increased so as to prevent each of these members of the rotary anode type X-ray tube from being deformed by the centrifugal force. However, if the rigidity of each of these members is increased, the size and the weight of each of these members are increased so as to give rise to the problem that the entire apparatus is rendered bulky.
In the rotary anode type X-ray tube disclosed in U.S. Pat. No. 5,838,763, both sides of the stationary shaft are supported by and fixed to a pair of supporting-fixing portions mounted in a vacuum envelope. The stationary shaft is fitted into the cylindrical rotary structure having a heavy rotary anode mounted thereto, and the rotary shaft is supported, by dynamic slide bearings arranged between the rotary shaft and the stationary shaft, in such a manner that the rotary shaft is rotated around the stationary shaft.
In this rotary anode type X-ray tube having a both-side supported beam structure, which is disclosed in the U.S. Patent quoted above, the stationary shaft is coupled to a vacuum envelope by supporting-fixing sections mounted at both edges of the stationary shaft. In this structure, the centrifugal force generated during the rotation of the X-ray tube around the subject to be diagnosed is dispersed to the pair of the supporting-fixing sections so as to decrease the deformations of the pair of the supporting-fixing sections and the stationary shaft. It follows that the defocusing of the electron beam is prevented. Also, the particular structure permits increasing the natural frequency so as to obtain a stable rotation even if the number of rotations per unit time is increased, compared with the structure disclosed in Japanese Patent No. 3,139,873 in which the rotary structure is rotated and mounted on the side of the free edge of the stationary shaft. It follows that, according to the both-side supported beam structure disclosed in U.S. Pat. No. 5,838,763, it is possible to increase the number of rotations per unit time of the rotary anode so as to obtain the merit that the temperature on the focal plane of the anode can be lowered.
In the both-side supported beam structure, however, a desired degree of parallelism between the stationary shaft and the cylindrical rotary structure is collapsed by the centrifugal force F acting on a heavy rotary anode, with the result that the cylindrical rotary structure tends to fail to be rotated smoothly. Also, since the stationary shaft is supported by a pair of supporting-fixing sections, the stationary shaft is deformed in a manner to depict a displacement curve having a single peak between the two supporting-fixing sections, if the centrifugal force is applied to the rotary structure. As a result, depending on the position of the peak of the displacement curve, the degree of parallelism between the stationary shaft and the cylindrical rotary structure is rendered poor in the bearing region in which a radial bearing and a thrust bearing are to be formed. As a matter of fact, a partial contact is brought about between the stationary shaft and the cylindrical rotary structure so as to give rise to, for example, seizing. It follows that the reliability of the bearing is lowered.